Quinone diazide positive-working photoresists, and similar positive-working compounds used in the preparation of lithographic printing plates, are described in U.S. Pat. No. 4,464,461, issued to Guild on Aug. 7, 1984, particularly from column 3, line 39 to column 7, line 16. The foregoing patent is hereby incorporated herein by reference. Commercial photoresists of this kind include OFPR-800; other photoresists sold by the Dynachem division of Morton International, Inc., Tustin, Calif.; and products sold by Shipley Company, Inc., Newton, Mass.; Eastman Kodak Company, Rochester, N.Y.; and others.
A positive-working photoresist functions by being coated on a suitable substrate, image-wise exposed to actinic radiation, then subjected to a development process which removes those portions of the photoresist which were previously exposed to radiation, leaving the unexposed portions of the resist intact. The developed photoresist pattern protects the corresponding portions of the substrate, from a further operation performed on the substrate, such as ion implantation, etching, plating, or the like. (In the case of printing plates, the residual portions of the photoresist have a different affinity for ink than the exposed portions of the substrate.) The known developers for quinone diazide positive-working photoresists comprise an aqueous solution of an alkali. The concentration of alkali is chosen to provide a developer which will selectively attack the exposed portion of the photoresist under the exposure and development conditions which have been selected.
While some commercially available developers contain metal salts such as sodium carbonate, sodium hydroxide, and others as alkaline agents, the art has recently chosen to avoid metal ion containing alkaline materials, or other metal ion sources, in photoresist developers. A concern has developed that residual metal ions left by the developer might form conductive paths in the finished device. Because of this avoidance of metal ion containing developers, the preferred alkaline materials are tetraalkylammonium hydroxides, and particularly tetramethylammonium hydroxide (TMAH). TMAH based developers are discussed in U.S. Pat. Nos. 4,423,138, issued to Guild on Dec. 27, 1983; 4,464,461, issued to Guild on Aug. 7, 1984; European Patent Application 0,062,733, filed by Cawston et al on Jan. 28, 1982 and published on Oct. 20, 1982, based on a corresponding U.S. patent application filed Apr. 10, 1981; Grieco et al, "Photoresist Developer Compounds", IBM Technical Disclosure Bulletin, Volume 13, Number 7 (December, 1970); "Improved Resist Developer," Research Disclosure 22713, March, 1983, pages 98-99; and others.
Several prior patents show the possibility of using morpholine or an alkanolamine, particularly ethanolamine, in photoresist developers. According to its English language abstract, Japanese patent application 59-119105, believed to have been published Dec. 26, 1985, teaches the use of either an inorganic alkali or an organic amine such as monoethanolamine or ethylenediamine as an alkaline agent in a photoresist developer. U.S. Pat. No. 4,464,461, column 1, lines 40-43, indicates that developers containing, for example, alkanolamines are "well known". U.S. Pat. No. 4,530,895, issued to Simon et al on Jul. 23, 1985, at column 1, lines 59-62, suggests use of a developer containing an alkaline substance such as diethylamine, ethanolamine, or triethanolamine as a photoresist developer. U.S. Pat. No. 4,411,981, issued to Minezaki on Oct. 25, 1983, discloses from column 3, line 46 to column 4, line 8, the use of a developing and etching solution for a photoresist containing various organic bases such as TMAH, monoethanolamine, diethanolamine, or triethanolamine, among many other basic reacting compounds. The Minezaki patent also discloses a photoresist developer in column 5, lines 10-16, and Table 3 comprising an aqueous solution of 5% TMAH, 1-2% morpholine, 0.04% coumarin, about 0.1% of an unspecified surfactant, and the balance water. Minezaki states this composition is diluted double or triple with distilled water to provide a developing and etching solution. The ratio of TMAH to morpholine is from 1:0.4 to 1:0.2 in this composition. None of these references suggests any reason to mix a quaternary ammonium compound and an alkanolamine or morpholine to correct any shortcoming of either material used alone as a developer.
As will be shown in comparative examples, TMAH used alone as a photoresist developer causes what appears to be a deposit of flaky residue along the upper and lower edges of lines of developer photoresists, particularly high contrast photoresists. The presence of this residue in exposed areas (which are intended to be free of photo-resist) suggests potential problems.
Alkanolamines by themselves are not suitable as developers for high contrast photoresists of the type exemplified herein, as they develop lines with poor resolution, fail to develop them altogether, or strip the photoresist. High concentrations of these developers also roughen the upper, normally smooth surfaces of developed photoresist lines.
One continuing challenge, as circuit geometries shrink and quality standards are maintained or raised, is how to maximize line width uniformity. Good line width uniformity means that lines of developed photoresist have nearly the same nominal line width and other dimensions as the mask lines and that these dimensions don't vary significantly depending on the location of the line on the wafer, the location of the wafer in a boat in which a batch of wafers are immersion processed together, or the order in which wafers are spray processed.
Another continuing challenge in photoresist developer research is how to achieve the desired line width uniformity despite variations from the nominal conditions selected for development. A developer composition which achieves this is said to have wide process latitude.
It is further desirable that a photoresist developer not cause the side walls of the developed photoresist to become less vertical (sloped).